Search results
Results from the WOW.Com Content Network
Changes of pore water content due to drying or wetting processes cause significant volume changes of concrete in load-free specimens. They are called the shrinkage (typically causing strains between 0.0002 and 0.0005, and in low strength concretes even 0.0012) or swelling (< 0.00005 in normal concretes, < 0.00020 in high strength concretes).
Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces is prone to creep. The density of concrete varies, but is around 2,400 kilograms per cubic metre (150 lb/cu ft). [1]
A w/c ratio higher than 0.60 is not acceptable as fresh concrete becomes "soup" [2] and leads to a higher porosity and to very poor quality hardened concrete as publicly stated by Prof. Gustave Magnel (1889-1955, Ghent University, Belgium) during an official address to American building contractors at the occasion of one of his visits in the ...
The minimum value of creep rate that is commonly applied to alloys is based on two norms: (1) the stress required to produce a creep rate of 0.1%/h × 10 −3 and (2) the stress required to produce a creep rate of 0.1%/h × 10 −4, which takes roughly about 11.5 years. The former standard has widely been used in the component design of turbine ...
A fairly well-defined reaction front can often be observed in thin sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and the microstructure of concrete are modified. These changes may vary in type or severity but commonly include: Extensive cracking; Expansion
The concrete can develop high compressive and tensile strengths, while shrinkage and creep remain acceptable, but will generally be less rigid than conventional mixes. The most obvious advantage is the low density, but these concretes also have low permeability to water and greater thermal insulation.
ASTM C1293: "Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction". It is a long-term confirmation test (1 or 2 years) at 38 °C in a water-saturated moist atmosphere (inside a thermostated oven) with concrete prisms containing the aggregates to be characterised mixed with a high-alkali ...
Concrete structures immersed in water as dams and bridge piles are therefore particularly sensitive. These reactions are also characterized by slow reaction kinetics, depending on environmental conditions such as temperature and relative humidity. They develop at a slow rate and may take several years before damages become apparent.